It has recently become possible to introduce cloned DNA fragments into the genome of Drosophila. The transformation system, developed by Gerry Rubin and Alan Spradling, was demonstrated using DNA from the rosy locus as a marker. The rosy locus, which codes for xanthine dehydrogenase, has been the subject of ongoing genetic and molecular studies in our labs, and we hope now to use the transformation technique to extend the analysis. Transformed flies with single new insertions of wild type rosy locus DNA be assayed for the absolute levels and developmental specificity of xanthine dehydrogenase expression. If most transformants look normal, the rosy DNA will be altered in-vitro at sites of interest, and the altered copies will be assayed by transformation. These experiments should define the DNA sequences of the rosy locus responsible for proper developmental expression.